An OED may be a device exhibiting its functions through exchange of charges between an electrode layer and an organic material. The OED may be an organic light emitting device (OLED), an organic solar cell, an organic photo conductor (OPC), or an organic transistor.
Conventionally, an OLED, which is a representative OED, sequentially includes a substrate, a first electrode layer, an organic layer including an emitting layer, and a second electrode layer.
In a structure known as a bottom emitting device, the first electrode layer may be formed as a transparent electrode layer, and the second electrode layer may be formed as a reflective electrode layer. In addition, in a structure known as a top emitting device, the first electrode layer may be formed as a reflective electrode layer, and the second electrode layer may be formed as a transparent electrode layer.
Electrons and holes are each injected by the two electrode layers, and recombined in the emitting layer, resulting in generating light. The light may be emitted to the substrate in the bottom emitting device, or to the second electrode layer in the top emitting device.
In the structure of the OLED, indium tin oxide (ITO) generally used as the transparent electrode layer, an organic layer, and a substrate which is conventionally formed of glass, have refractive indexes of approximately 2.0, 1.8, and 1.5, respectively. In such a relationship of the refractive index, for example, the light generated in the organic emitting layer in the bottom emitting device is trapped at an interface between the organic layer and the first electrode layer or in the substrate due to total internal reflection, and only a very small amount of light is emitted.